Machines are known which include the use of windowless envelopes, obtained starting from a strip of paper wound on a reel on a sprocket without containing edges.
The strip of paper includes, in succession, the printed signs for each envelope and the markers for the separation of the signs.
The reel is loaded in a feeder associated to the machine and the band is sent to an envelope-forming station; the envelopes are then transferred to an insertion station where the line carrying the stacks of sheets to be introduced into each envelope leads.
Each envelope, either before or after the introduction of the relative stack, crosses a printing station which prints thereon the address of the addressee, according to a list memorised in a command board associated to the station.
The above-described machines, thanks to the high operating speed and the low costs of the envelopes thus-presented, are preferred for sending non-personalised content, where each addressee receives the same materials, for example advertising, informative and the like.
In this case the only distinctive element between one envelope and a next is the address printed on the envelope; thus it is not necessary to check that one stack of material is inserted rather than another.
The situation is different when the correspondence is personalised, for example for sending bills, bank current accounts and the like, in which it is necessary for the material to be sent to a particular addressee.
In theory, if the sequence of the prepared stacks matches that of the stored addressees, the correct combination could be achieved, but any problem that might arise would lead to a staggering of the lists, with all the ensuing risks that would bring.
For these types of operations, envelopes bearing at least a transparent window are preferred, as the address on the stack can be seen and read; in this way any errors of combination between the envelope and the contents will be revealed.
Machines are known which use the envelopes with windows already prepared, in which the supplier of the envelopes comprises a store, where the envelopes are arranged in piles, and extracting organs which collect them one at a time from the pile in order to send them to the insertion station.
These machines have a limited autonomy, with respect to the cited ones, due to the small capacity of the envelope store, in order to enable an operator to load the store of the feeder without stopping the machine.
Apart from the constant presence of an operator, it is clear that these envelopes with windows have a considerably higher cost than those obtained from a reel, as they have to be realised in a special paper workshop, inserted in boxes, stored and transported to the place of use.
Consequently costs for despatch for correspondence made using the above-mentioned envelopes with windows will be greater.
At present paper reels for envelopes with windows are not available, due to a series of drawbacks, and also due to the additional thickness of the transparent acetate film and the glue, which creates zones of discontinuity in the thickness.
In order correctly to understand the problem, consider that a normal reel of paper for envelopes without windows has a diameter of about 1200 mm and ensures an autonomy of about 30,000 envelopes.
The thickness of the paper usually used is, for example, 0.11 mm, which becomes 0.15 in the frame surrounding the window, where the paper and the acetate film overlap, with an interposing of glue.
When winding a strip of paper with a window and relative acetate the windows are staggered along the winding circumference but are transversally aligned.
The staggering along the winding circumference almost totally compensates for the increase in thickness of the sides of the transversal frames of the reel, which in each new spire are in a new position, different from that of the previous spire.
The transversal sides, therefore, are mutually intercalated and are cyclically at the centre of the underlying windows, where only the thickness of the acetate film is present; the combination of these two factors limits the increase of the development of the circumference of each spire given the increased thickness of the transversal sides of the frame.
The sides of the frame arranged parallel to the winding direction, being wider than the winding direction and aligned transversally, lead to a formation in the reel of two stripes having a greater thickness with respect to the other zones, due to their progressive superposing.
The winding of the spires is consequently tight only at the stripes, while it is substantially loose in the external sides of the reel.
A reel exhibiting the above-described characteristics must necessarily have a limited diameter, less than half with respect to normal reels, in order to contain the inevitable lateral movements which occur, with an operating autonomy reduced to about 5000 envelopes.
Further, the reel is unstable if rested on the ground and creates difficulties in all the manoeuvres for its movement preceding use thereof.
The lack of lateral compactness of the spires makes the edge of the band vulnerable; they can get damaged, resulting in poor-quality envelopes.